Reworkable Underfill Evaluation for Fine Pitch BGA Applications

ABSTRACT Interest in using fine pitch SMT components has increased greatly in recent years due to the growth of portable, hand held electronics and due to miniaturization trends in consumer and industrial electronics markets. The reliability of those fine-pitch portable electronics products is a great concern particularly in the areas of impact and shock performance. For very fine pitch SMT components such as CSPs and BTCs without ground planes (0.5mm pitch or lower), underfills can be used to improve the impact and thermal cycle reliability. Historically, the target properties of underfills can be generally summarized as high glass transition temperature (Tg), high modulus (E) and matched coefficient of thermal expansion (CTE) to solder. However, the underfill selection and evaluation process has become increasingly complex, time consuming and cost prohibitive due to increasing product design constraints, introduction of new package materials, and ever changing from factor of semiconductor packages. With every new generation of package technology, one must factor into the underfill selection process, new solder alloys and soldermasks, thinner substrate core materials, finer pitches, and increasing package dimensions. This paper presents the process evaluation and reliability results for four commercial reworkable underfills. The study centers on fine-pitch SMT underfill applications using large 0.5mm pitch BGA components. Seven criteria were investigated which included a materials property comparison (viscosity, Tg, modulus and cure), underfill flow rate (70°C and room temperature), flux compatibility, reworkability, Solder extrusion, thermal cycle reliability, and drop/impact reliability. The evaluation results showed that three underfills had similar high performance and are comparable. The other material had lower performance in all six criteria. However, that material performed best in reworkability.